1. Field of the Invention
The present invention relates to a sewing machine for synchronously controlling a main shaft and a loop taker with separate motors.
2. Description of the Related Art
A conventional ordinary sewing machine mainly includes a bed portion, a column portion, an arm portion, and a head portion. In the head portion, a needle bar, a sewing needle, and a needle thread take up lever are vertically driven by a driving force of a main shaft. In the bed portion, a loop taker is driven by a lower shaft to cooperate with the sewing needle to form thread loops with a sewing thread. According to a sewing machine of an ordinary type, the lower shaft is driven also by the main shaft because the loop taker has to be moved synchronously with the sewing needle.
There has been proposed sewing machines of another type that employs a special loop taker drive motor for driving the loop taker independently from the main shaft. In this type of sewing machines, the loop taker can be moved synchronously with the main shaft. Minute-by-minute rotational states of the loop taker can be precisely controlled according to a sewing condition.
For example, Japanese Examined Patent Application Publication (Kokoku) No. SHO-60-21750 proposes a sewing machine provided with a needle drive motor for driving a sewing needle and a loop taker drive motor for driving a loop taker. The sewing machine drives the two motors synchronously with each other so that the sewing needle and the loop taker operate synchronously to each other to sew a perfect seam.
Japanese Laid-Open Patent Publication (Kokai) No. SHO-61-15816 proposes another sewing machine that controls synchronization between the needle drive motor and the loop taker drive motor, thereby preventing skipped stitches and improving the tightness of the stitches.
Japanese Laid-Open Patent Application Publications (Kokai) Nos. HEI-3-234291 and HEI-3-234293 provide other sewing machines that maintain interlocking operations between the sewing needle and the loop taker. In each of these sewing machines, a sewing motor is provided to drive the sewing needle via the main shaft. A loop taker driving motor is provided separately and independently from the sewing motor. The loop taker driving motor is for driving the loop taker. A rotary encoder is provided to the main shaft for detecting the rotational amount of the main shaft. When the main shaft is rotated manually, the loop taker drive motor is automatically rotated exactly the amount the main shaft has been rotated. Accordingly, the interlocking relationship between the loop taker and the sewing needle is maintained.
The above-described sewing machines with independently driven main shaft and loop taker is designed so as to detect the origin of the main shaft, the rotational angle of the sewing motor, the origin of the loop taker, and the rotational angle of the loop taker drive motor. Detection signals indicative of the detected results are used to control the sewing motor and the loop taker drive motor to rotate them synchronously with each other.